1. Field of the Invention
The present invention relates to a polygonal mirror device for an optical scanning used in an image forming apparatus for optically forming an image, such as a laser printer.
2. Description of the Related Art
FIG. 2 is a cross sectional view showing a conventional polygonal mirror device.
As shown, a polygonal mirror 1 is formed around an outer peripheral surface of a rotor member 2, and the rotor member 2 is rotatably fitted to a shaft 4, which is mounted to a housing 3a. A specific groove is formed on the shaft 4, to thereby form a hydrodynamic gas bearing which generates a dynamic pressure when the rotor member 2 rotates.
The rotor member 2 includes a motor magnet 5. A stator core 6, a coil 7 and a motor board 8 are mounted on the housing 3a to face the motor magnet 5. The coil 7 is magnetically excited to rotate the rotor member 2.
And a magnetic attraction force is generated between the motor magnet 5 and the stator core 6. At this time, the rotor member 2 poises at a position where the attraction force balances a weight of the rotor member 2. A dynamic unbalance of the rotor member is corrected by cutting a surface C and a surface D of the rotor member 2, such as a small drill, whereby generation of mechanical vibration of the rotor member 2 is suppressed to maintain a stable rotation of the rotor member 2. Holes 10 as cutting traces are left at specific positions on the surfaces C and D.
The rotor member 2 is covered with the housing 3a and a housing 3b as well, thereby preventing reflecting faces 1a of the polygonal mirror 1 from being soiled with dust and the like. A light beam goes into and out of the polygonal mirror device, through a window (not shown) formed in the housing 3b, in order to form a scan beam of light by the polygonal mirror 1.
As described above, the conventional art corrects the dynamic unbalance of the rotor member by cutting the surface C and the surface D of the rotor member 2 as correcting portions by means of a drill, whereby generation of its mechanical vibration is suppressed to maintain a stable rotation of the rotor member 2. Therefore, the holes 10 as cutting traces are left at specific positions on the surfaces C and D. Because of a centrifugal load generated by the rotation of the rotor member 2, the rotor member 2 is deformed in the radial direction. When the holes 10 are nonuniformly present on the rotor member 2, the rotor member 2 is also nonuniformly deformed in the radial direction. A height of the surface C is equal to that of the reflecting faces 1a of the polygonal mirror 1 measured from the same. Accordingly, the reflecting faces 1a of the polygonal mirror 1 formed on the outer peripheral surface of the rotor member 2 are also deformed nonuniformly, so that the reflecting faces 1a are strained by the centrifugal load. As a result, an accuracy of the light beam deflection for scanning is degraded, and when the polygonal mirror device is applied to, for example, a laser beam printer, this leads to a variation of a scanning angle by the rotation of the polygonal mirror device, and to deterioration of the print quality of the printer.
When the surface C is bored by the cutting means, the resultant hole 10 infrequently passes through the rotor member 2 and reaches the surface E located opposite to the surface C. When the through-hole reaches the surface E, the surface E is burred. The rotor member 2 is rotated at extremely high speed of 40,000 rpm. If the burr is left as it is, the burr is separated from the surface E by a centrifugal force, and will reach the reflecting faces 1a of the polygonal mirror 1, thereby possibly damaging the reflecting faces 1a. This brings about degradation of an accuracy of the light beam deflection for scanning. When it is applied to a laser beam printer, for example, a problem of deterioration of the print quality of the printer arises. To cope with this, it is necessary to remove the burr of the surface E by use of a reamer or a drill bit of which the diameter is larger than the diameter of the through-hole.
In the structure of the polygonal mirror device, the motor magnet 5, the stator core 6 and a ring portion (motor portion) including a coil are arrayed near the surface E. Accordingly, the surface E and its vicinal region are more complicated in configuration than the surface C and its vicinal region. This makes it difficult to remove the burr. For this reason, some of the burr remains on the surface E.
In FIG. 2, the diameter of the inner surface 11 of a portion of the rotor member 2, which is located near the surface E and to be abutted against the motor magnet 5, is larger than the inside diameter of the motor magnet 5. Therefore, when after the boring, the surface E and its vicinal region are cleaned by use of a compressed-air blowing machine, such as an air gun, part of cutting particles produced when the rotor member 2 is cut remains on the surface E and its vicinal region. If the burr and the cutting particles remaining on the surface E and its vicinal region are left, the burr and the cutting particles are separated from the surface E by a centrifugal force, and will reach the reflecting faces 1a of the polygonal mirror 1, thereby possibly damaging the reflecting faces 1a. This brings about degradation of an accuracy of the light beam deflection for scanning. When it is applied to a laser beam printer, for example, a problem of deterioration of the print quality of the printer arises.
Attempt to make it easy to remove the burr on the surface E will limit a freedom in designing the motor portion including the stator core 6 and the coil 7. Further, when the work of correcting the dynamic unbalance is done on the surface C, and successively it is done on the surface D, it is required to invert the rotor member 2 or to use a couple of opposed cutting means. As a result, the number of manufacturing steps or cost of related equipment is increased.